On Sunday 26 October 2008 06:06:07 am Jérôme Gardou wrote:
I was wondering why directsound was not implemented using openal. As direct3d is implemented on top of opengl.
It's not very easy to get DirectSound working on top of OpenAL. I actually made a DSound->OpenAL wrapper (as a native C++ Win32 DLL, so it can't go into Wine as-is even if I wanted it to), but it's none too stable. Some apps work, others don't. Some work better than others.
The biggest problem is updating the sound buffer in real-time (DSound lets you lock a sound buffer and modify it while it's playing; OpenAL only lets you rebuffer the whole buffer, while it's not attached to any source). I get around this by spawning a real-time thread which queues small chunks "just in time" to keep the buffer playing, but this is a hack at best.. there's no way to know how much you need to buffer ahead so OpenAL doesn't run out during an update, some apps are more sensitive to a large write-ahead, and real-time threads aren't really available in Wine (meaning even larger sections would need to be queued to be able to update in time). The problem is compounded when you consider multiple DSound buffers can share the same data buffer.
Another problem is the lack of panning. OpenAL has no controls for panning a 2D sound. At most you can move the sound left and right to simulate panning (for mono sources; stereo is out of luck), but the actual algorithms are implementation dependant and thus would not produce consistant results.
Additionally, OpenAL hides a lot of the hardware vs. software caps. OpenAL doesn't tell you if you're using hardware or software, while DSound would tell you. Of course you can always lie and say you have hardware (which I found was needed to get some games to even attempt to use DSound3D), but it's not entirely proper.